The theremin is an electronic instrument, was invented by a young Russian Physicist, Leon Theremin in October of1920. The theremin was originally the product of Soviet Government sponsored research into proximity sensors. How does this relate to music? In the electromagnet field, when an object passes through, it emits a sound that changes the pitch of the field. The further away the object is the lower the sound, and the closer the object is in the field, the higher the pitch.For an instrument that you do not physically touch or hold this become a key thing in playing the instrument. The instrument has two antennas which protrude from the theremin. One straight antenna that controls the pitch, and the other is a curved antenna which controls the volume. Each hand does two different actions that are independent from each other. When working in tandem, they help to produce an electrical lyrical sound.
Here is a link of Leon Theremin himself playing the Theremin: https://youtu.be/_3H5JbkPXpw